1. Field of the Invention
The present invention relates to an improved heat-developable photosensitive material having a photosensitive layer which contains in its binder a reducible organic silver salt being able to isolate silver from it by the action of energy externally applied thereto.
2. Description of the Prior Art
In the technical fields of information recording and printing, a rapid and remarkable advancement has been made in recent years in developing novel methods and materials used therefor which allow a simple and accurate recording and printing at a higher speed. Among them, for example, heat-developable photosensitive material has become one of the objects of particular attention in the art. Unlike the conventional photographic photosensitive material consisting essentially of inorganic silver salt, the novel heat-developable photosensitive material consisting essentially of organic silver salt as image forming agent does not necessitate any wet treatments, or developing and fixing after imagewise exposure. These novel types of photosensitive materials are, for example, disclosed in U.S. Pat. Nos. 3,457,075, 3,531,286 and 3,589,903 specifications.
These materials are usually formed by applying on a suitable support such as plastic film, plastic sheet or paper a photosensitive layer comprising an organic silver salt employing a suitable technique such as coating or immersion.
As mentioned above, the use of this type of material in forming an image has such merit that all the processes required to form an image may be carried out with dry treatments. In spite of such valuable advantage, however, there are still several problems to be solved in these novel materials.
In order to make it possible to obtain always high quality images, the uniformity in grade, distribution and selection of the every component substance of the photosensitive material as well as in quality of the produced material is of great importance. Moreover, it is required that, after formed, the material be hardly rendered to a change on standing.
Furthermore, there are other requirements related to the image forming process which such material should satisfy. These requirements are deduced from the fact that the novel material is heat-developable photosensitive material which is subjected to a heat-developing treatment after an imagewise exposure so as to form an image thereon. To carry out the heat-development, conduction heating means such as a pair of heating rollers is generally employed. Therefore, to produce a high quality image, the heat for heat-developing should be uniformly transmitted to all over the area of the heat-developable photosensitive material. On the other hand, the material per se must be a material of relatively high heat conductivity enough to allow the uniform transmission of heat for heat-developing. However, this can not be satisfied completely by the known heat-developable photosensitive materials. Usually, the heat-developable photosensitive material is in the form of a sheet comprising a support such as paper and a photosensitive layer applied thereon, which photosensitive layer is typically formed by dispersing in a suitable binder such as resin an organic silver salt as essential component. This structure of material often causes an irregular transmission of heat to be used for heat-developing the exposed material. Sometimes there is formed an unevenly developed image.
Speeding up the image forming process using the heat-developable photosensitive material is another problem. Since the heat-developing step is a rate-determining step in the image forming process, if a substantial speed up of the heat-development be attained, it will contribute to a great extent to the desired reduction in time necessary for the image forming process. But, it is difficult to realize that. When conduction heating means such as a pair of heating rollers is used for effecting heat-developing, some modifications of the heating means may be required to speed up the heat-development while keeping the heat conductivity of the heating means at a certain desired level. Firstly, in order to prevent the heating rollers from being cooled by the sheet of heat-developable photosensitive material passing through therebetween, it is required to enlarge the radius of the heating rollers so that the contact time with the sheet per one revolution may be shortened. Secondly, it is required to increase the calorific power of heat source such as electric heater mounted in the roller. These requirements will be satisfied only by a considerable enlargement in size of the heat-developing apparatus and a substantial increase in electric power consumption which are of course disadvantageous.
A further problem arises when such heat-developable photosensitive material is used for producing an electrostatic printing master disclosed in our prior application, U.S. Ser. No. 599,061. Namely, when various steps from a master forming step to an electrostatic printing step are carried out using such heat-developable photosensitive material in a continuous and automatic process and all the necessary treatments are carried out on a printing support such as drum, there may be caused some troubles by heat-developing with the conduction heating means. Like the above mentioned case, a considerable enlargement in size of the heat-developing apparatus and a substantial increase in electric power consumption are necessary to attain a speed up of the master forming step. Since the printing support such as drum on which the sheet of heat-developable photosensitive material is to be placed, is generally made of metal, the heat for effecting the heat-development is apt to dissipate in the printing support. As a result, the sheet of heat-developable photosensitive material is insufficiently heated so that there may occur insufficiency or irregularity in heat-developing. Moreover, even when a visible image of relatively good quality is produced, there may occur the case that the master formed therefrom lacks an adequate electrostatic characteristics which is essential for a good printing master. On the contrary, if a large amount of heat is supplied as to sufficiently heat-develop the sheet of heat-developable photosensitive material, there may be caused another trouble, apart from the disadvantage of the need of a greater amount of heat. The trouble is that the temperature of the printing support rises and thereby an elevation of the temperature of all the printing apparatus may be caused. This will adversely affect other parts or equipment such as the electric circuit parts.
A further problem involved in electrostatic printing using such master is found in the structure of the heating means. In order to obtain a maximum of efficiency of thermal conduction, the conduction heating means used in this printing process is usually the contact type which is brought into contact with the photosensitive sheet laid on the printing master during heat-development. At other stages of the process, in particular, during an electrostatic printing stage, the contact type of conduction heating means should be brought to a position apart from the photosensitive sheet so as to prevent the heating means from obstructing the electrostatic printing. Such conduction heating means makes the heat-developing apparatus equipped with it mechanically complicated and apt to get out of order. Moreover, it necessitates a considerable space for which the heating means moves from its contact position to its removed position and vice versa. This militates against the purpose of making the apparatus compact as a whole.
Under these conditions, it is concluded that an infrared heating means can be more effectively and more advantageously used as heat developing means, in particular, in performing the above described electrostatic printing. Compared with the known contact type of heating means, the infrared heating means which is of non-contact type has many advantages such as a high heat-developing efficiency, a capability of uniform heat-developing, a compact structure of apparatus and a relatively low consumption of heat.
However, there has not yet been proposed any heat-developably photosensitive material comprising reducible organic silver salt and having a good heat-developing property suitable for a heat-development employing infrared heating means.